Most people know Ehrlich as the environmentalist who brought the world's attention to the overpopulation problem in the 1960s. But this Stanford biologist has also enjoyed a long, eminent career exploring evolution. In his new book, he combines his scientific research and environmental concerns into an enlightening narrative of humanity's evolution. Ehrlich surveys the most important research on the origin and rise of hominids and current ideas about the ascent of language and consciousness. He accepts that we are the products of evolution, but he finds the current trends of evolutionary psychology and genetic determinism to be hopelessly simplistic. Instead, Ehrlich shows how genes, culture and the environment together create a complexity that, he says, science still barely grasps. The 100,000 or so genes in human DNA, he contends, could never determine the 100 trillion connections between the neurons in our brains. Evolution may shape our brains generically, but the culture and environment in which we grow up control its fine details. Moving into the more recent past, Ehrlich charts how cultural (rather than biological) evolution has created civilizations, and how it has later destroyed many of them. Finally, he shows how an understanding of human evolution can inform our ethics and our decisions about how to run our societies. It shows, for instance, that under their skin, all humans are practically identical genetically speaking; we cannot pretend that race has any biological significance. We still have a long way to go from an evolutionary point of view: our ancestors spent millions of years living in small groups and dealing with the immediate struggle of finding food, and we have not yet adapted to the globalized society or such problems as human-created climate change. Although Jared Diamond and others have plowed this ground before, Ehrlich's book is so well researched and so elegantly presented that it stands as one of the best introductions to human evolution in recent memory. And that along with Ehrlich's name recognition should help this break out from the usual. science audience. 20,000 first printing; 8-city author tour; national radio interviews; national print advertising. (Oct.)